Climate change may influence infectious disease dynamics through a wide range of pathways, underscoring the importance of mechanistic studies to evaluate the net effects of temperature on pathogen transmission as well as host pathology.  As temperatures warm, pathogen development rates and transmission may be enhanced, but this effect may also be counter-acted by improved immune responses in ectothermic hosts. We studied the influence of temperature on the transmission of the trematode parasite, Ribeiroia ondatrae, as well as its pathology within chorus frog hosts, Pseudacris regilla.  We exposed 60 tadpoles to 25 R. ondatrae cercariae at three temperatures (17, 20, and 26 °C) to assess short-term transmission success.  We exposed an additional 96 tadpoles to 28 R. ondatrae to measure the long-term effects of temperature on host pathology and infection levels at metamorphosis.

Results/Conclusions

Both parasite transmission efficiency and tadpole immune response were enhanced at 26°C.  Higher temperature significantly elevated initial parasite transmission efficiency with an average of 66% of parasites penetrating tadpoles in the 26°C treatment compared to 53% in the 17°C treatment.  The warmer temperature also improved tadpole immune response such that a significantly lower proportion of parasites that penetrated tadpoles were able to successfully encyst inside the tadpole. While there were no differences in survival among treatments at metamorphosis, parasite-induced deformities in the 20°C treatment were 4 times more prevalent than the 26°C treatment, and 1.4 times more prevalent than the 17°C treatment.  Infection of early-stage hosts increases the likelihood that they will develop malformations.  Thus the observed differences in malformation frequency are likely due to the earlier average gosner stage of tadpoles at cooler temperatures over the 9 day infection period, while the lower parasite transmission efficiency at 17°C may have reduced pathology in that treatment.  Because host developmental stage has such a strong influence on the prevalence of deformities, climate-driven changes in the timing of interactions between R. ondatrae and its tadpole hosts in the field could have strong consequences for the resultant pathology.  Experiments demonstrating the effect of temperature change on each component of the transmission dynamics of complex host-parasite systems can begin to clarify the net effect of climate change on disease dynamics and host pathology.